Electronic structures of antiperovskite superconductors Mg X Ni 3 ( X = B , C, and N)

We have investigated electronic structures of a newly discovered antiperovskite superconductor ${\mathrm{MgCNi}}_{3}$ and related compounds ${\mathrm{MgBNi}}_{3}$ and ${\mathrm{MgNNi}}_{3}.$ In ${\mathrm{MgCNi}}_{3},$ a peak of very narrow and high density of states is located just below the Fermi level, which corresponds to the ${\ensuremath{\pi}}^{*}$ antibonding state of Ni $3d$ and C $2p$ but with the predominant Ni $3d$ character. The prominent nesting feature is observed in the $\ensuremath{\Gamma}$-centered electron Fermi surface of an octahedron-cage-like shape that originates from the 19th band. The estimated superconducting parameters are in reasonable agreement with experiment, suggesting that the superconductivity in ${\mathrm{MgCNi}}_{3}$ is described properly by the conventional BCS phonon mechanism.